The present invention relates to moving picture coding and decoding technologies, and more particularly, to moving picture coding and decoding technologies using predictive coding of quantization parameters.
In digital moving picture coding according to MPEG-2 Part 2 (hereinafter, referred to as MPEG-2), MPEG-4 Part 10/H.264 (hereinafter, referred to as AVC), or the like, a picture is divided into blocks of a predetermined size and is coded, and a quantization parameter that represents the roughness of the quantization with respect to a predictive error signal (or simply referred to as a picture signal) is transmitted. On the coding side, by performing variation control of the quantization parameter in units of predetermined blocks, the amount of coding can be controlled, and the subject picture quality can be improved.
As control of the quantization parameter for improving the subjective picture quality, adaptive quantization is frequently used. In the adaptive quantization, the quantization parameter is changed in accordance with the activity of each block such that fine quantization is performed for a flat portion in which deterioration is more easily noticeable visually, and rough quantization is performed for a complex pattern portion in which deterioration is not easily noticeable visually. In other words, the quantization parameter is changed such that a large quantization scale is set in a macro block having a high activity level in which the amount of assigned bits after coding may be easily increased. As a result, the subjective picture quality is improved while the number of bits of coded picture data is controlled to be as small as possible.
According to the MPEG-2, it is determined whether or not the quantization parameter of a previous block in the sequence of coding/decoding and the quantization parameter of a coding target block are the same. In a case where the quantization parameters are not the same, the quantization parameters are transmitted. Meanwhile, according to the AVC, differential coding of the quantization parameter of a coding target block is performed by using as a prediction value the quantization parameter of the previous block in the sequence of coding/decoding. The reason for this is that, generally, since the coding amount control is performed in the coding sequence, the quantization parameter of the previous block in the coding sequence tends to be closest to the quantization parameter of the coding block, and this is for suppressing the information amount of the quantization parameter to be transmitted.
Japanese Patent Application Laid-Open No. 2011-91772
In a conventional control process of the quantization parameter, a difference between the quantization parameter of a coding target block and the quantization parameter of a coded block, which is disposed on the left side thereof, serving as a predictive quantization parameter is calculated, and the calculated differential quantization parameter is coded, whereby the coding amount of the quantization parameter is decreased. However, depending on the content within the screen, for example, as illustrated in FIG. 8, in a case where the characteristic of a picture disposed inside the coding target block and the characteristic of a picture disposed inside a left block, which has been coded, are different from each other, a difference between the quantization parameters that are calculated by the adaptive quantization process is large, and accordingly, even when a prediction of a quantization parameter in relation with the left block is uniquely made, the differential quantization parameter is large, whereby there is a problem that the coding amount increases.
In addition, since the quantization parameter calculated in the coding amount control process is acquired in the raster scanning order from the upper left side to the lower right side of a normal screen, when the block size of the coding target is small, the processing sequence between slices is separated away. Accordingly, in a case where the quantization parameter of a coded block that is neighboring to the upper side of the coding target block is used for a prediction of the coding target block, although the upper block is neighboring to the coding target block, the processing sequence in the coding amount control process is separate from each other, and accordingly, there is no probability that the quantization parameter calculated in the coding amount control process necessarily has the same value or a close value between the coding target block and the coded block neighboring to the upper side. Therefore, there is a problem in that the coding amount of the differential quantization parameter cannot be determined to be reduced.